I. Field of the Invention
The present invention relates generally to shroud assemblies for turbine engines and, more particularly, to such a shroud assembly with means for maintaining tip clearance for the turbine blades over the engine operating temperature range.
II. Description of the Prior Art
Historically, improvements in turbine engine performance have been heretofore achieved by increases in the gas temperature at the turbine inlet. Current projections anticipate a continuing increase in the operating temperatures for the turbine while maintaining or even improving component efficiencies and the overall efficiency of the engine.
High turbine engine efficiency requires a minimization of the turbine rotor tip clearance, i.e. the clearance between the outer radial ends of the turbine blades and the turbine rotor shroud over the operating temperature range of the engine. Due to the thermal expansion of the turbine disc, the turbine blades and the shroud, turbine rotor tip clearance control and minimization becomes increasingly difficult as the turbine inlet temperatures increase.
With the previously known turbine engines, the rotor tip clearance is preset to a predetermined value, for example 0.05 inches for a 6.3 inch radius turbine rotor when the engine, and consequently the engine components, are cold. During rapid engine start and acceleration, both the engine shroud and turbine blades rapidly reach their operating temperatures and, as a result, thermally expand. The thermal expansion of the shroud, however, exceeds that of the turbine blades so that the rotor clearance increases to, for example, 0.12 inches in the given example at engine start up.
As the turbine reaches a steady state operation, the disc in addition to the shroud and blades also reaches its operating temperature thus reducing the rotor tip clearance to, for example, 0.06 inches for the given example. This relatively wide rotor tip clearance at the steady state operating condition for the turbine engine substantially adversely affects the overall turbine engine efficiency.
During a throttle chop, i.e. when the turbine engine is rapidly shut down, both the shroud and turbine blades rapidly cool and thus thermally contact. The turbine disc, however, retains its heat for a relatively longer period of time and thus remains in a state of thermal expansion of, for example 0.04 inches for the 6.3 inch radius turbine rotor. It is this thermal expansion of the disc which establishes the assembly tip clearance requirement in order to prevent siezure of the turbine during engine shut down.
There are a number of previously known methods designed to reduce the rotor operating tip clearance, and thereby increase engine efficiency, during operation of the turbine engine. These previously known methods include segmenting the shroud and supporting the shroud from relatively cool rails. Similarly, an increase in the cooling air flow across the shroud has been used to decrease the thermal expansion of the shroud and thus decrease the rotor tip clearance. These previously known methods, however, each have their own undesirable characteristics and, therefore, are only partially effective in operation.